The invention relates to a field-frequency doubling circuit, comprising a converter circuit for converting an interlaced television signal, having a picture frequency of n pictures per second and a field-frequency of 2n fields per second, into a converted television signal having a field frequency of 4n fields per second, the fields of the converted television signal having substantially the same number of lines as have the fields of the interlaced television signal to be converted and every two consecutive fields of the converted television signal being equal to each other, and comprising, coupled to an output of the converter circuit, an interpolation circuit for adding together information from some of the lines of the converted television signal in each line period of the converted television signal.
U.S. Pat. No. 4,435,728, which corresponds to Netherlands Patent Application 8100603 (PHN 9947), discloses a field-frequency doubling circuit of the above-defined type, which comprises a movement detector which, in the absence of movement switches the circuit to a type of conversion in which the fields of the converted television signal alternately correspond to a first and a second field of the television signal to be converted. The interpolation proposed for the case in which there is movement, relates to combining information from two consecutive lines of a field of the converted television signal.